Adding texture to one side of a mesh only ( based on axis)
The Normal link in a Geometry node is dedicated for this purpose:
In the Node Editor, add the Geometry node, link the Normal to a Separate XYZ node, then use the Z link for a factor in a mix node. In my example I added a Color Ramp node to control This factor, check the image below for the nodes setup, and the Gif to see how you can use the Color Ramp node to control the effect.